On your Mac, simply launch Rivet. Once its icon turns black you're all set. On your Xbox 360 go to the "My Xbox" row and select one of the media items to go into that media section. You will then be presented with a list of available media sources like the hard drive and DVD drive. Near the bottom of the list you should have an item called <your mac> : Rivet. Select it. Your Xbox will then display the media being shared from your Mac with Rivet.